1. Field of the Invention
The present invention concerns insulating refractory products having controlled porosity as well as the preparation thereof by the wet method.
2. Description of the Prior Art
It is known that the manufacturing of devices for casting, purifying and handling molten metals and more particularly corrosive molten metals such as aluminum, is conditioned by the perfectioning of refractory materials capable simutaneously of bearing the high temperature at which these molten metals are kept, of withstanding effectively their very corrosive action and lastly of bearing the thermal shocks to which the walls of these devices are subjected. During the last few years, various efforts have been made with a view to perfecting such materials. In general, these efforts have made it possible to obtain refractory products of the ceramic type which withstand well the aggressiveness of metals. But in numerous cases, these products are not very resistant to thermal shocks. Now, it is known that the resistance of a material to thermal shocks varies in an inverse proportion to the modulus of elasticity of the material, which itself depends directly on its porosity. It therefore appeared that a method enabling the controlling of the porosity of a refractory product during the manufacturing thereof gives the means of defining its resistance to thermal shocks.
Very recently, a process enabling the controlling of the porosity of a product of the type comprising ceramic substances obtained by sintering has been perfected. It consists more particularly in mixing a powder of oxydes of Zirconium, Hafnium, Yttrium, Lanthanum, Beryllium and Calcium, with granulated substances resulting from the crushing of a dense sintered product obtained from these same powders, then in sintering that mixture at a relatively low temperature in a fluoride of a metal of group I of the periodic classification of elements. The porosity depends hence on the proportion of granulated products incorporated with the powder. The results obtained are very satisfactory but the method proves to be expensive and the industrial application thereof is probably fairly limited.
Moreover, processes using the wet process seemed not to have any future. Indeed, when concrete is made by the adding of a hydraulic binding agent, there is no mastery of the porosity obtained. By vibrating the product placed in a mould for a more or less long period, the porosity thereof may at the most be decreased but not increased.
Indeed, processes for manufacturing refractory products formed by mixing in a stirring machine products such as tabular alumina and aluminous cement by the damp method are known, but in these known methods, the product thus obtained is fired at a temperature in the order of 1700.degree. C., leading to a great sintering of the product. That process therefore leads to a refractory ceramic substance which does not withstand well repeated thermal shocks.
Other manufacturing processes using the wet process, with the same refractory products and effecting the firing at lower temperatures, are also known, but in these processes, the sintering of the product is made easier by incorporating a flux such a bentonite which increases, indeed, the cohesion of the grains of the refractory product but which reduces, at the same time, the porosity of the finished product and, subsequently its resistance to thermal shocks, lastly, by the incorporating of the flux, a product having a poor resistance to erosion and corrosion by reactive metals such as aluminum is formed.
There were also proposed in U.S. Pat. Nos. 3,294,562 and 3,269,849 refractory compositions to be used in contact with molten nonferrous metals of melting points below about 1000.degree. C., such as aluminium and aluminium base alloys, consisting essentially of 40 to 60% by weight of asbestos fibers, 30 to 20% by weight of calcium aluminate cement, 30 to 20% by weight of a fibrous material selected from the group consisting of fibrous alumina-silica which is refractory and substantially inert toward molten aluminium and fibrous potassium titanate, pressed under a high pressure and fired at temperatures from about 1400.degree. F. to 2000.degree. F. The disclosed alumina-silica fibers were either fibers with substantially equal amounts by weight of alumina and silica, such as the Fiberfrax and Kaowool fibers, or fibers of substantially pure silica, such as the Refrasil material. However, the asbestos fibers become brittle upon dehydration between 400.degree. C. and 600.degree. C., and the alumina-silica fibers have a high silica content and are therefore subject to corrosion by molten aluminium, which progressively reduces their silica content into silicium, and moreover such compositions have a relatively low resistance to fracture under thermal shock, so that they withstand a relatively low number of successive immersions into, and emersions from, molten aluminium, and cannot be used in devices to be submitted to contact with molten aluminum during long service periods, such as pumps for molten aluminum.
The search for refractory products simultaneously meeting the requirements of resistance to corrosion and erosion by molten aluminum and aluminum base alloys and of resistance to thermal shock, and capable, moreover, of being obtained on an industrial seale has led us to define a new product as refractory as ceramic substances but withstanding repeated thermal shocks and very long contact with molten aluminum.